The present invention relates generally to method for the removal of pollutants from water. More specifically, the present invention relates to methods for biological conversion of hydrogen sulfide in solution to sulfate ion in high salinity ground waters and industrial wastewaters.
Environment-conscious industries are continuously laboring toward the goal of removing pollutants from contaminated water to make the water safe at both the ground level and the consumer level. Government-regulated agencies establish limits for many common industrial pollutants. These limits tend to become stricter as pollutant reduction and removal technology proves effective at accomplishing previously-established requirements. Consequently, both ground and consumer level water continue to improve in terms of both purity and safety.
Among the methods employed to reduce and remove pollutants, bioremediation constitutes an effective and desirable technology. In a broad sense, bioremediation includes the use of microorganisms that digest pollutants as a source of food, including nitrogen and carbon compounds. Bacterial metabolism can convert the pollutants to metabolites having a simple chemical structure to carbon dioxide and water in an aerobic process, or to methane in an anaerobic process. In any respect, the metabolites produced by bacteria typically have no adverse environmental effects.
The use of large volumes of water in hydraulic fracturing or “fracking,” a technique to enhance the recovery of natural gas from organic containing shale deposits, has led to restrictions on use of fresh water reserves both above ground and from potable ground waters. The use of non-potable deep ground water brines for fracking does not put a strain on these fresh water reserves, but it does introduce new challenges, especially with regards to high levels of hydrogen sulfide present in brines. Several chemical processes have been developed, including chemical oxidation or stripping/adsorption techniques for sulfide removal from waste water, but they tend to be relatively expensive and require an undesirably large amount of time, machinery and high operational costs. Sludge disposal is one of major operating costs of biological treatment systems. The use of sulfur oxidizing bacteria that produces elemental sulfur in a suspended growth system produces large amounts of solids that contribute to sludge formation of both elemental sulfur solids and biological solids.
The oxidation of sulfides by the chemolithotrophic bacteria in a bioreactor to remove sulfide requires oxygen. The oxygen is generally provided by sparging air directly to the bioreactor. However, air sparging can result in extensive stripping of hydrogen sulfide out of the water phase before the bacteria are biochemically oxidize the sulfide to either elemental sulfur or soluble sulfate. Additionally, the stripped sulfide requires extensive scrubbing from the vent gas. Therefore, in an attempt to reduce stripping of sulfide during biological treatment of high sulfide groundwater or wastewater, a low aeration is utilized. But low aeration favors biological conversion of sulfide to insoluble elemental sulfur rather than sulfide due to low dissolved oxygen levels in bioreactor. The formation of elemental sulfur is problematic as it requires separation and solids handling as its disposal. The build-up of elemental sulfur can lead to plugging problems in the bioreactor resulting in poor sulfide removal and a greater proportion of sulfide in the off-gas stream.
Therefore, there is a need for an improved method and apparatus for removing sulfides and their conversion in a cost and time efficient manner. It is also desirable to provide such methods and systems that can replace some conventional chemical processes for removal of sulfides with improved biological processes that produce less sludge and thereby reduce the requisite time, machinery, and operational costs for performing the processes.